Tsunamis are typically produced by either abrupt surface-floor physical displacements (e.g., subsea earthquakes or landslides) or atmospheric anomalies (e.g., the latter are called meteo-tsunamis). So the first possible indication of a tsunami might be the seismic detection of an earthquake. However, not all subsea earthquakes produce tsunamis, and hence the magnitude of an earthquake cannot be used to forecast the generation or intensity of a resulting tsunami.
One type of sensor that sees and measures the intensity of a tsunami is a bottom pressure sensor connected to a buoy overhead. Developed by the National Oceanic and Atmospheric Administration (NOAA), networks of these sensors (called DART™ for Deep-ocean Assessment and Reporting of Tsunami) were deployed after the catastrophic 2004 Banda Aceh (Indonesia) earthquake whose subsequent tsunami claimed a quarter of a million lives. DART™ sensors observe the height of the tsunami wave as it passes above them. The tsunami height measured by these buoys is then inputted to numerical tsunami models to give rough forecasts of arrival and intensity at coastal points around the world. An example of such a numerical model is described in Implementation and testing of the Method of Splitting Tsunami (MOST) model, V. Titov and F. Gonzalez, NOAA Tech. Memo. ERL PMEL-112 (PB98-122773), NOAA/Pacific Marine Environmental Laboratory, Seattle, Wash., 11 pp, (1997), the entire disclosure of which is incorporated herein by reference for all purposes.
However, the DART™ network is still sparse, so that not all tsunamis can be observed and inputted to the model before coastal impact. Furthermore, because of the wide range of variation of the bathymetry (i.e., depth of water offshore) of different coastal regions, the model's forecast of timing and intensity at the coast is often only very coarse.